Such luminescent materials known from DE-B-23 63 744 are used in particular for color picture tubes. Cubic ZnS:Cu,Al (zinc blend, sphalerite) renders possible a higher luminous efficacy and a longer-wave emission than hexagonal wurtzite. Sphalerite is converted into wurtzite from 1020.degree. C. upwards. To prevent this, the luminescent materials mentioned above must not be heated to above approximately 1000.degree. C. during manufacture.
Such luminescent materials must have a high luminous emission power for a given intensity of the electron irradiation. Furthermore, x-values for the color point of at least 0.29 are to be aimed at, so that the luminescent materials after their application on a picture screen and the unavoidable reduction of the x-value occurring during this still have sufficient x-values (which according to the EBU standard is an x-value of approximately 0.28).
When dopants consisting of Cu and Al are employed, i.e. when exclusively Cu and Al are used as dopants, because it is desirable to avoid toxic Cd or Au which gives problems with manufacturing technology, it has been possible until now to introduce only comparatively small proportional quantities of Cu and Al if a still sufficient brightness were to be safeguarded. In that case, however, only comparatively small x-values at the lower limit of the admissible range were possible.